Spooky scary bloodsucking phantom voltage that disappears when a load is applied to the wires. There is a cool demonstration on a pro's site that shows 120V worth of induced voltage on a wire that is completely unattached from any power source. This is why solenoid type testers are the best, in my opinion for homeowners. They apply a small load (voltage makes them vibrate, often) and make the scary voltage go away.